Acromegalic patients have continued to be followed with respect to pituitary irradiation. Further, we are evaluating the effects of transsphenoidal hypophysectomy in these patients and comparing them to the pituitary-irradiated patients. Recently, human growth hormone has been cross-linked to its specific cellular receptor on IM-9 cultured lymphocytes. Under reducing conditions the predominant band is an approximate 140K protein. Under non-reducing conditions, however, a molecular weight component of about 270K is observed. Further attempts were made to see if the growth hormone receptor was a protein kinase or a substrate for this kinase activity; it is not. Thus, the growth hormone receptor is not analogous to the insulin receptor. The heterogeneity of circulating growth hormone in plasma has been studied. Pituitary growth hormone was injected in normal volunteers and the individual growth hormone components isolated by gel filtration. It was found that the half-time of the "little" (22,000 Dalton) growth hormone component was faster than the "big" and the "pre-big" growth hormone components, respectively. This is compatible with a receptor-mediated type of removal of these components; previous studies have shown that the high molecular weight forms have lower radioreceptor activity than the 22,000 Dalton growth hormone preparation.